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Re: Verified evasion in Snort
Feb 01 2006 09:22PM
mwatchinski sourcefire com
This and other target base fragmentation evasions are the reason we re-wrote the fragmentation engine in Snort.
If you look at Judy Novak's Frag3 Development paper, Snort's latest fragmentation engine (frag3) supports target-based fragmentation policies for overlaps, ttl evasions, and timeouts. This can be configured on a per IP basis to allow exact emulation of how the end host handles fragmentation reassembly.
Here is a sample configuration that could be used for frag3. This configuration would handle the evasion outlined in the advisory. This configuration is based on the 5 second timeout used in the PoC code provided.
preprocessor frag3_engine: policy first bind_to 10.2.1.0/24 timeout 5 detect_anomalies
From our testing, Windows XP actually has a 1 minute timeout for fragments. The actual configuration to handle this evasion would be the following:
preprocessor frag3_engine: policy first bind_to 10.2.1.0/24 timeout 60 detect_anomalies
For the VRT's detailed analysis of the PoC tool and the advisory please see:
Director, Vulnerability Research
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